The present invention relates in general to information storage and retrieval systems, and more particularly, to a method and apparatus for providing branch metric compensation for sequence detection in partial response channels in a digital magnetic recording system.
In digital magnetic recording systems, data is recorded in a moving magnetic media layer by a storage, or xe2x80x9cwritexe2x80x9d electrical current-to-magnetic field transducer, or xe2x80x9cheadxe2x80x9d, positioned immediately adjacent thereto. The data is stored or written to the magnetic media along one or another of selected paths therein by switching the direction of flow of a substantially constant magnitude write current which flows through windings in the write transducer. Each write current direction transition results in a reversal of the magnetization direction in that portion of the magnetic media just passing by the transducer during current flow in the new direction, with respect to the magnetization direction in the media induced by the previous current flow in the opposite direction. In one scheme, a magnetization direction reversal over a path portion of the media moving past the transducer represents a binary digit xe2x80x9c1xe2x80x9d and the lack of any reversal in that path portion represents a binary digit xe2x80x9c0xe2x80x9d.
When data is to be recovered, a retrieval, or xe2x80x9creadxe2x80x9d magnetic field- to-voltage transducer, (which may be the same as the write transducer if both are inductive) is positioned to have the magnetic media, containing previously stored data, pass thereby such that flux reversal regions in that media either induce, or change a circuit parameter to provide a voltage pulse to form an output read signal for that transducer. In the scheme described above, each such voltage pulse due to the occurrence of a magnetization direction change in corresponding successive media path portions represents a binary digit xe2x80x9c1xe2x80x9d and the absence of a pulse in correspondence with no such change between successive path portions represents a binary digit xe2x80x9c0xe2x80x9d.
In digital magnetic recording systems using peak detection of such voltage pulses as the data recovery method to digitize the read signal, the times between voltage pulses are used to reconstruct the timing information used in recording the corresponding data previously stored in the magnetic media to define the path portions described above. More specifically, the output of such a peak detector is used as an input signal to a phase-locked loop forming a controlled oscillator, or phase-lock oscillator (PLO), or synchronizer, which produces an output clock signal from the positions of the detected peaks of the read signal. Absolute time is not used in operating the data retrieval system since the speed of the magnetic media varies over time which results in nonuniform time intervals between read signal voltage pulses.
A data encoding scheme known as run-length-limited (RLL) coding is commonly used to improve the PLO""s reconstructed clock signal accuracy based on avoiding drift in the frequency thereof because of too much time between voltage read signal pulses. When RLL code is employed, the time durations between read signal voltage pulse transitions is bounded, that is, the number of binary digits of value xe2x80x9c0xe2x80x9d that can separate binary digits of value xe2x80x9c1xe2x80x9d in the read signal is limited. This constraint is known overall as a (d, k) constraint where the individual constraint xe2x80x9cdxe2x80x9d represents the minimum run length of zeros, or the number thereof between ones, while the individual constraint xe2x80x9ckxe2x80x9d represents the maximum run length of zeros permitted. The xe2x80x9cdxe2x80x9d portion of the constraint can be chosen so as to avoid crowding of voltage pulses in the read signals which can reduce intersymbol interference problems in which portions of read signal voltage pulses overlap. By limiting the number of consecutive zeros, the xe2x80x9ckxe2x80x9d constraint maintains the reliability of the PLO in providing an accurate clock signal for the retrieval system. An automatic gain control (AGC) system is used to maintain signal amplitude for the PR4 channel, and the xe2x80x9ckxe2x80x9d restraint also maintains the reliability of the AGC.
In digital magnetic recording systems employing partial response (PR) signaling, which involves the acceptance of intersymbol interference, data recovery is achieved by periodically sampling the amplitude of the read transducer output signal, as initiated by clock pulses of the PLO, to digitize that signal. In this scheme, each clock pulse of the PLO initiates a sample which has a value contributed to it by more than one pulse in the transducer read signal. Accordingly, a partial response detection system for a PR channel is designed to accommodate the effects of such intersymbol interference, and therefore the xe2x80x9cdxe2x80x9d constraint may not be necessary (i.e. d=0). The xe2x80x9ckxe2x80x9d constraint is still necessary in PR signalling because the PLO is still used to provide timing for sampling the read signal, and because the AGC is used to maintain sample amplitude in connection with the PR channel.
A Class 4 PR channel, which is typically the selected frequency response chosen for the signal channel through which the read signal passes prior to detection thereof, is particularly suitable for magnetic recording with typical pulse characteristics because the channel requires very little equalization to achieve an overall match of this Class 4 response. In a Class 4 PR channel for typical pulse characteristics, signal samples are independent of their immediately neighboring samples, but are dependent on samples 2 clock samples away. The read samples are submitted to a Viterbi detector which generates the data that most likely produces the sample values. More particularly, the clock captures digital sample values using an analog-to-digital converter (ADC) where each sample value may be the summing result of more than one pulse read from the magnetic media. These samples are transformed by signal processing techniques to match certain target values. It is based on these transformed samples that the Viterbi detector or other sequence detector recovers the data.
Once a particular signalling scheme is chosen, the structure of the Viterbi detector is configured according to a state diagram for the signalling scheme. A state diagram in the form of a trellis is particularly suitable for a Viterbi detector since it incorporates the time element. An output/input relation is associated with each branch of the trellis. The target value based on an input for a particular branch of the trellis is known as the metric for that branch. A two state Viterbi decoder fits each of the time indexed sample values at the channel output with two allowable channel output sequences. One allowable output sequence minimizes the sum of squared errors over all possible noiseless output sequences ending in a first state of the trellis at time k. The other allowable output sequence minimizes the sum of squared errors over all possible noiseless output sequences ending in a second state of the trellis at time k. The Viterbi detector keeps track of the minimum cumulative branch metric for each state through the trellis over a predetermined time period for determining the path which best fits the sample data to the target values. A complete description of partial response channels, coding techniques and Viterbi detection is available, for example, in xe2x80x9cModulation and Coding for Information Storagexe2x80x9d by Paul Siegel and Jack Wolf, IEEE Communications Magazine, December 1991, at pages 68-86.
As mentioned above, partial response signalling is to equalize a voltage pulse read from the magnetic media to a certain target value and apply linear superposition to a combination of pulses in order to record more in a given area on the magnetic disc. Each target value of the read back signal from the disc is the linear sum of pulses considered in the signalling scheme. However, one of the problems in high density recording, such as in a partial response signaling, is that the read back waveform is not merely the linear sum of pulses. A magnetic transition on the media may be nonlinearly effected by adjacent transitions in both the write and the read processes so that the read back waveform is distorted. Since the amplitude and location of a read back pulse may be nonlinearly distorted by any nearby magnetic transitions, it is desirable to provide a detection scheme which takes into account and corrects for such magnetic distortions.
The present invention provides a sequence detection scheme which takes into account amplitude and/or time and/or other magnetic distortions caused by neighboring magnetization regions on the magnetic medium, wherein the distortions in one magnetization region are caused by the closeness of neighboring magnetization transitions on one or both sides thereof. The detection scheme according to the present invention provides an extended state diagram to include the effects of leading (past) and/or trailing (future) magnetization transitions. More particularly, it has been found that accounting for the effects of trailing transitions requires an increase in the number of states in the state diagram used to form the Viterbi detector. Leading transitions are neutralized by increasing the number of branches between states in the Viterbi detector. Increasing the number of branches instead of states keeps the complexity low and thus saves hardware and associated costs.
According to one embodiment, the data sequence detector according to the present invention is employed for a Class 4 partial response channel for use in a digital magnetic recording and read-back system. In such a system the sequence detector recovers an estimated sequence of bits of binary data [a1, . . . , ak, . . . , an] which most likely corresponds to an original sequence of bits of binary data [A1, . . . , Ak, . . . , An]. The original sequence is precoded into a write sequence of bits of binary data [B1, . . . , Bk, . . . , Bn] which are represented by magnetization regions on a magnetic medium. The estimated sequence of bits of binary data is recovered from a waveform read back from the magnetic medium and transformed into a digital waveform [X1, . . . , Xk, . . . , Xn].
To account for the amplitude and/or time distortions in one magnetization region which are caused by the closeness of neighboring magnetization transitions, the data sequence detector is configured according to a predetermined trellis model having eight (8) current states identified by bits (Bkxe2x88x922, Bkxe2x88x921, Bk) of the write sequence, eight (8) next states identified by bits (Bkxe2x88x921, Bk, Bk+1) of the write sequence, at least two (2) branches diverging from each current state, at least two (2) branches merging into each next state, a target value Yk for each branch of the trellis for use in a branch metric calculation for selecting one of a pair of the branches, and at least two (2) bits of the estimated sequence associated as an output for each state of the trellis.
To account for the amplitude and/or time distortions in one magnetization region which are caused by the closeness of neighboring magnetization transitions, the data sequence detector is configured according to a predetermined trellis model as mentioned above, modified such that there is provided four (4) branches diverging from each current state, four (4) branches merging into each next state, an additional bit Bkxe2x88x923 of the write sequence identifying one of a pair of the four branches, and three (3) bits of the estimated sequence associated as an output for each state.